Electrical prevention of deposition of solids



United States ELECTRICAL PREVENTION 6F DEPOSITION F SQLIDS Filed Get. 28, 1957, Ser. No. 692,525

No Drawing.

Claims.

This invention relates to treatment of alkali metal hydroxid solutions. It more particularly involves cooling aqueous sodium hydroxide solutions.

In the manufacture of alkali metal hydroxides, it is frequently necessary to cool alkali metal hydroxides, e.g., in the course of concentrating alkali metal hydroxide solutions. For example, electrolysis of brine solutions provides dilute sodium hydroxide solutions, say of 8 to 15 percent sodium hydroxide by weight. For many purposes, these sodium hydroxide solutions are too dilute. Hence, they are concentrated by evaporating a portion of the water. The resulting hot concentrated aqueous sodium hydroxide solution must be cooled to purify further the material and to provide a product which, is conveniently stored or shipped.

When the temperatures of aqueous sodium hydroxide solutions are reduced by the use' of cooling coils or other like heat transfer techniques, serious problems are encountered. One of the most troublesome is the deposition of solids on the cooling surface in contact with the solution.' So extensive is this deposition that within several days cakes several inches in thickness build up on the cooling surface and the coolers efiiciency' drops noticeably. In less than a Week, the cooler no longer functions with efliciency justifyingcontinued use, and its operation is advisedly halted for cleaning. The cake must be removed by washing with water which requires that the coolers be taken out of service.

It has been discovered that aqueous alkali metal hydroxide solutions which when hereto-fore cooled by heat transfer techniques deposited a solid cake on the heat transfer surface now may be cooled by heat exchange in a manner which minimizesor substantially eliminates such deposition. Coolers may be operated-in accordance with this invention for much longer periods of time and with markedly increased heat transfer characteristics. In addition, the frequency of shutdowns and maintenance problems are reduced.

In accordance with the present invention, it has been discovered that the above described advantages and other benefits are realized by cooling hot aqueous alkali metal hydroxide solutions in contact with an anodic (or positively charged) heat transfer surface. Thus, by passing a direct electrical current of low voltage and low amperage between a negative electrode electrically insulated from the heat transfer surface and immersed in the aqueous alkali metal hydroxide, and the positively charged heat transfer surface, there is a striking reduction in the rate of solids deposition on the heat transfer surface. This has resulted in substantial improvement in the heat transfer characteristics of the coolers. Coolers not so operated while cooling hot hydroxide solutions evidence a sharp decline in their cooling capacities, often decreasing by more than two-thirds in the cooling capacity in one or two days.

Not only are the above enumerated advantages realized, but surprisingly enough, corrosion of the cooling surface and the electrolysis of the sodium hydroxide or like if 31st alkali metal hydroxide solutions is at .rnost' negligible or inconsequential.

Aqueous sodium hydroxide solutions, especially those containing; upwards of 25 percent and below percent sodium hydroxideby weight, at temperatures of from about 150 F. to 250? F. are cooled in this manner by passage through a cooler containing cooling coils or like heat transfer surfaces, a coolant such as process water being circulated through the coils. The coolant is normally 25 F..to F. cooler than the solution being cooled. That is, there is a temperature differential of 25 F. to 80 F. across the cooling surface. In the operation of the cooler, a direct electrical current is passed between an electrode immersed in the aqueous sodium hydroxide solution (but otherwise electrically insulated from the cooling surface) undergoing cooling and the heat transfer surface, e .g., the surface of the cooling coils. The heat transfer surface itself is positively charged and the electrode negatively charged. The voltage and amperage of the electrical current is variable. An average current density of 0.05 to 1.0 amperage per square foot of heat transfer surface provides effective protection against deposition of solids and minimizes the reduction in heat transfer efficiency of the cooling surfaces. Depending upon the surface areas of the negative electrode and the heat transfer surface, a direct electrical current of from 1 to 10 volts and 1 to 70 amperes is used in providing for the average current densities.

This process offers pronounced advantages in conjunction with the cooling of aqueous alkali metal hydroxide solutions and notably aqueous sodium hydroxide solutions which otherwisedeposit solids" on the cooling surfaces. Solutions which are particularly prone to dep i oli t e cooling su f c nd th rwi b inefii' en ly coole re ho C nta ing sa s, mm nly; pr sen in. elec r y c l y p odu od um hydroxide, such asfsodium chloride, sodium carbonate and sodium sulfate or the like, The precise salt concentration O any one or all of the salts at which deposition and other disd an age are o rve w l d p ng p n. he temperatures of the solution prior to and subsequent to ool ng. How r, in c njunct on wit op n practices, aqueous sodiurnhydroxide' solutions containing at least about 1 percent sodium chloride by weight or at least bou per ent o t es a t by gh no ma y counter" the problems herein ob'viated. v

In h main, q o s. s dium hy oxid lu ions like alkali metal hydroxide solutions, containing from about 25 percent to 75 percent sodium hydroxide by Weight and at least 1 percent (usually 5 to 25 percent) by. weight of a salt or salts such as sodium chloride, sodium bicarbonate and sodium sulfate which are cooled from temperatures in the range of '150" F. to 250 F. down to 70 F. to F. are those which encounter the most severe difliculties of solids deposition and reduced heat transfer characteristicsof the coolers, Thus, it is in conjunction with the cooling of such aqueous alkali metal hydroxide solutions that the present invention is most appropriately used, offering considerable incentive in the way of operating advantages and economy.

Any number of coolers operated inseries may be employed in the cooling of aqueous alkali metal hydroxide solutions. Although advisable, it may not, be necessary to positively charge the heating surfaces in each cooler. Positively charging the heat transfer surfaces of the first or first few coolers in a series frequently suffices to protect those heat transfer surfacesfnormally encountering the greatest deposition of solids; In the, other coolers in the series, the dePQSitiOn may at times not be of sufficient magnitude o J t y p s tive charg n of heir he t t an fer. lut-aces.

In the u e of a p ural yof s ale s Series it has been found that aqueous sodium hydroxide solutions at from about 2 to 250 F. may be cooled in the initial cooler or the first two coolers to a temperature on the order of 170 F. without encountering undue solids deposition on the positively charged heat transfer surfaces while the subsequent coolers need not be electrically protectedQ Further cooling to 70 F. to 100 F. in subsequent coolers of the series is not plagued with serious solids deposition. Hence, positively charging the cooling surfaces in these coolers is less important.

The precise configuration of electrodes disposed in the cooler and immersedin the liquid may take many shapes and forms depending upon the shape of the cooler, the

shape and disposition of cooling surfaces and whether the agitators are in the cooler. While it is advantageous to design the negative electrode such that it is equidistantly EXAMPLE Six coolers were connected in series providing for the continuous flow of aqueous sodium hydroxide through coolers 1 through 6 sequentially,'the hottest sodium hydroxide being introduced into cooler 1. Countercuzrent to the flow of caustic, cooling water was passed through the cooling coils in each cooler. Thus, the coldest cooler water passed initially into the coils'of cooler 6. The coolers including cooling coils were of nickelto eliminate contamination caused by contact of exposed surfaces with the hot aqueous sodium hydroxide. Each cooler was equipped with an agitator.

Coolers 1 and 2 were comprised of cylindrically shaped containers having a diameter of 84 inches and a vertical axis of about 9 feet. In each of these coolers, the cooling coils provided 349 square feet of cooling surface and were comprised of four-inch outer diameter twelve gauge nickel tubes; Cylindiically shaped coolers" 3 and 4 were 108 inches in diameter'and about 10 feet high. Each of these coolers had 652 square feet of cooling. surface provided by cooling coils of four inch outer diameter. Coolers 5 and 6 were comprised of cylindrically shaped containers having an .internal diameter of 108 inches and a vertical height of about 9 feet. Each of coolers 5 and 6 were equipped with coils of four-inch outer diameter twelve gauge nickel having 5 68 square feet of cooling surface.

Aqueous sodium hydroxide having a composition as follows:

- Percent by weight. NaOI-I 50.50. Nafl 2.81 NfizSO; Na CO 0.31

was fed at the rate and temperature indicated in Table I to the above described cooling system with the coolers being operated in series; e.g., the caustic was fed continuously and initially into cooler 1, then to cooler 2 and so on through cooler 6. The coolant circulated through the cooling coils was water at the temperature indicated days of operation in each instance.

Temperature of NaOH Solution Leaving Coolers 85 8 Temperature of Cooling Water Feed F.) 54. 5 53.9

Temperature of Cooling zyaFter Leaving Coolers Gallons of Water per Ton of NaOH Solution (containing 50 percent NnOH by Weight) 468 290 In run A, no electrodes were immersed in the coolers and the cooling coils were not electrically charged. In

run B, an electrode which was electrically insulated from the coolingcoils, agitator and the shell of the cooler was immersed in the liquid contents of the cooler and an electric'current of2.9 volts and 60 amperes was passed between the cooling coils and the electrode with the cooling coils being positively charged.

In run C, the cooling coils were operated as in run B except that the direct current was at 3 volts and 60 amperes. In run D, both coolers 1 and 2 had immersed therein electrodes electrically insulated from the cooling In run E, coolers 1 and 2 were operated with immersed electrodes as in run D. A direct current of 3.9 volts and 60 amperes was fed between the cooling coils and electrode in cooler 1 .while a direct current of 3.9 volts and 34 amperes was fed in cooler 2 between the cooling 1 coils and immersed electrode. In run F, coolers 1 and 2 were equipped as in runs D and E with the direct current in cooler 1'being2.'9 volts and amperes and the direct current in cooler 2 being 2.9 volts and 20 amperes.

As can be readily seen from the data tabulated in Table I above, the passing of electrical current between positively charged cooling coils and the immersed cathodes materially improves the cooling elficiency of the cooling system. Thus, in run A (where the cooling coil-s were not electrically charged) the gallons of water? required to cool a ton of aqueous sodium hydroxide.

was substantially greater than the other runs in which the cooling coils in either coolers 1 and -2 were positively charged.

1 Furthermore, the heat transfer coefficients of the cooling coils in coolers 1 and 2 strikingly reflect the improvements realized by positively charging the cooling coils and passing electric currents between the cooling coils and an electrode immersed in the aqueous sodium hydroxide solution undergoing cooling. Thus, in run A when no current was passed, the heat transfer coeificients (in British thermal units/hour/square foot/degree Fahrenheit) of the cooling coils in cooler I dropped from about 50 to about 25 after but several days of operation. In a similar fashion, the heat transfer coetficients of the cooling coils in cooler 2 dropped from about to 20 after five days of operation.

On the other hand, the heat transfer coefficients of cooling coils in coolers 1 and 2 when the cooling coils were positively charged and a current passed between them and the immersed electrode remained fairly con "stant. For example, in run E, the heat transfer coefficient of the cooling coils in cooler 1 after six days of operaasenasa Furthermore, visual observation of the cooling coils subsequent to shutdown of the cooling operation showed clearly the deposition of solids around the cooling coils was strikingly minimized and almost completely avoided by positively charging the cooling coils and passing a current between the cooling coils and the immersed electrodes. Thus, while deposition of solids around the cooling coils in run A after six days was such that a covering on the order of 1 to 3 inches in thickness was found, in runs B through F there was a noticeable decrease in the thickness of the deposition on the coils. For example, in run E, the cooling coils in coolers 1 and 2 had very little solids deposited thereon. While in run A there was a continuous thick coating of solids, in run B only scattered portions of the cooling coils had any deposition whatsoever.

It is good practice that all equipment coming into contact with the aqueous alkali metal hydroxide be fabricated of materials which do not contaminate the alkali metal hydroxide or corrode under the operational conditions. Nickel and nickel alloys are advised for this reason. Thus, the cooling coils, inner surface of the coolers and the immersed electrodes are advisedly of nickel or nickel alloys.

While the present invention has been described with reference to specific details of certain embodiments, it is not intended that it be construed as limited to such details except insofar as they are included in the appended claims.

We claim:

1. In the cooling of an alkali metal hydroxide solution without altering significantly the alkali metal hydroxide content of the solution due to the cooling by removal of heat through a heat transfer surface wherein deposition from said solution of solids on the heat transfer surface in contact with said solution normally occurs, the improvement which comprises positively charging electrically the heat transfer surface and passing a direct electrical current between the surface and a negative electrode immersed in the aqueous alkali metal hydroxide whereby deposition of solids on the heat transfer surface is retarded.

2. The method of claim 1 wherein the alkali metal hydroxide is sodium hydroxide.

3. The method of cooling an aqueous alkali metal hydroxide solution without altering significantly the alkali metal hydroxide content of the solution due to the cooling which comprises contacting the solution with a heat transfer surface, positively charging electrically the heat transfer surface and passing a direct electrical current between the heat transfer surface and a cathode immersed in the aqueous alkali metal hydroxide solution whereby to retard the deposition from the solution of solids on the heat transfer surface in contact with the solution which otherwise normally occurs.

4. In the cooling of an aqueous sodium hydroxide solution without altering significantly the alkali metal hydroxide content of the solution due to the cooling by heat transfer through a heat transfer surface to a coolant wherein deposition from the solution of solids on the heat transfer surface in contact with the solution normally occurs, the improvement which comprises positively charging electrically the heat transfer surface and passing between a cathode immersed in the aqueous sodium hydroxide and the heat transfer surface a direct electrical current having an average current density of 0.05 to 2.0 amperes per square foot of heat transfer surface whereby to retard the deposition of solids on the heat transfer 'ride on the heat transfer surface in contact with the solution which otherwise normally occurs.

References Cited in the file of this patent UNITED STATES PATENTS 1,227,453 Kipper May 22, 1917 1,825,477 Reichart Sept. 29, 1931 2,526,878 Kay Oct. 24, 1950 

1. IN THE COOLING OF AN ALKALI METAL HYDROXIDE SOLUTION WITHOUT ALTERING SIGNIFICANTLY THE ALKALI METAL HYDROXIDE CONTENT OF THE SOLUTION DUE TO THE COOLING BY REMOVAL OF HEAT THROUGH A HEAT TRANSFER SURFACE WHEREIN DEPOSITION FROM SAID SOLUTION OF SOLIDS ON THE HEAT TRANSFER SURFACE IN CONTACT WITH SAID SOLUTION NORMALLY OCCURS, THE IMPROVEMENT WHICH COMPRISES POSITIVELY CHARGING ELECTRICALLY THE HEAT TRANSFER SURFACE AND PASSING A DIRECT ELECTRICAL CURRENT BETWEEN THE SURFACE AND A NEGATIVE ELECTRODE IMMERSED IN THE AQUEOUS ALKALI METAL HYDROXIDE WHEREBY DEPOSITION OF SOLIDS ON THE HEAT TRANSFER SURFACE IS RETARDED. 